Battery separators are well known in the art and comprise microporous membranes interjacent adjacent plates (i.e., electrodes) in a stack of plates forming a cell element. Such separators serve to separate, and electrically isolate, one plate (e.g., positive electrode) from the next (e.g., negative counterelectrode) and have sufficiently small pores to suppress interelectrode dendrite growth, but sufficiently high porosity to permit electrolyte mobility within the cell. Typical such materials comprise sintered PVC particles or microporous polyethylene such as described in Baylor et al Pat. No. 3,551,210 or Larson et al Pat. No. 3,351,495 respectively. Such battery separators include a plurality of ribs on the face thereof to space the separator from one of the plates (i.e., usually the positive plate). The separator may be used as individual pieces which are merely interspersed between the several plates or in the form of envelopes wrapped around one of the plates. In the latter instance, a sheath for the plate is formed by folding a length of the separator sheet about the bottom edge of the plate and heat sealing the lateral edges of the sheet together to form an envelope.
Lead-acid batteries, among others, are known to be susceptible to electrolyte concentration variations from top to bottom. This problem is particularly acute during the recharge of deep discharge batteries. In this regard, the heavy H.sub.2 SO.sub.4 formed at the plates during recharge settles to the bottom of the battery causing higher concentrations of H.sub.2 SO.sub.4 thereat than at the top of the battery. This electrolyte concentration imbalance is known as electrolyte stratification and is detrimental to battery performance and life. It has been proposed to avoid electrolyte stratification by pumping the electrolyte from the bottom of the battery to the top thereof to promote better concentration uniformity. Some such proposals are shown in Sundberg, U.S. Pat. Nos. 3,083,253; Sundberg, 3,305,404; Halsey 734,549; Scholl et al 4,194,060 and Newell 4,469,759. Such proposals are complex solutions to the problem. It would be much more desirable to prevent, or at least minimize, stratification in the first instance, particularly if it could be accomplished in a simple cost effective way.
It is accordingly an object of the present invention to provide an improved battery separator having ribs on the face thereof designed and arranged so as to substantially avoid the accumulation of high concentrations of sulfuric acid at the bottom of the battery. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.